1. Field of the Invention
The present invention relates to a hand-held drive-in power tool for driving in fastening elements and including a guide, a drive-in ram displaceable in the guide, drive means for driving the drive-in ram and including at least one preload drive spring, and a tensioning device for preloading the drive spring.
2. Description of the Prior Art
Hand-held power tools of the type described above are used for driving fastening elements in a constructional component with the ram. The drive spring serves as a driving source and is preloaded by a tensioning device. The advantage of the above-described tool consists in that the mechanical drive spring can be economically produced which permits to insure a cost-effective manufacturing of the entire power tool. Further, an advantage of mechanical springs over gas springs in general consists in that upon preloading of a mechanical spring, the temperature does not increase as it takes place in gas springs. As a result, a preloaded mechanical spring does not lose the stored energy for a long time, whereas in a gas spring, the stored energy is gradually lost because of leakage.
However, mechanical springs have a drawback in comparison with gas spring that consists in that upon a rapid expansion, a substantial portion of the energy, which is stored in the spring becomes lost as it has to be used for accelerating the spring mass proper. Because the mass of a mechanical spring is much greater than the mass of a gas spring, these losses are much greater than in the gas spring. As a drive-in process that takes place with the drive-in power tool, which is subject of the present invention, leads to a very rapid expansion of the spring, the foregoing circumstance is very noticeable.
A drive-in power tool of the type discussed above is disclosed in German Publication DE 40 13 022 A1. The disclosed power tool includes a spring for driving an impact mechanism toward the tool mouth for driving in a nail. The device for displacing the impact mechanism in its initial position includes an electric motor and a speed reduction mechanism. The rotation of the electric motor is transmitted by the speed reduction mechanism and a crown gear, which forms part of the speed reduction mechanism, to a hammer body of the impact mechanism for displacing the impact mechanism against the biasing force of the drive spring into the initial position in which the impact mechanism is ready for a drive-in process.
The drawback of the drive-in power tool of DE 40 13 022 A1 consists in that the maximal impact energy that can be applied by the spring to the hammer body, about 5-10 joules, is somewhat low. Therefore, such a drive-in tool cannot be used for driving in fastening elements in hard constructional components, such as steel and concrete. This is the result of the above-discussed circumstance that the mechanical spring loses a portion of the stored energy for acceleration of the spring mass, so that this portion is lost for acceleration of the impact mechanism. If the impact energy of the drive-in tool is to be increased, a stronger spring should be used which would store more energy. However, the increase of the spring strength leads to an increase of the spring mass which, in turn, again increases the losses which result from a portion of the energy being spent on the acceleration of the spring mass.
This means that the energy, which is stored by the springs, should be increased in order to increase the setting or drive-in energy. This, in turn, results in a significantly heavier spring, without a noticeable increase of the impact energy of the drive-in power tool.
Accordingly, an object of the present invention is to provide a drive-in power tool in which the drawback of known drive-in power tools are eliminated.
Another object of the present invention is to provide a drive-in power tool in which with simple technical means, a high drive-in energy, together with a high drive-in speed, are achieved.